The present invention relates to pulsed light sources such as a pulsed laser light generating apparatus, and more particularly to control of the output energy of such pulsed light sources.
Recently, laser apparatus having an oscillation frequency in the ultraviolet range have attracted attention as pulsed light sources for fine machining. Of these laser apparatus, an excimer laser can provide strong oscillation beams with several wavelengths in a range of 353 nm to 193 nm with halogen gas such as fluorine and chlorine being combined with noble gas such as krypton and xenon as the laser medium. This excimer laser is one of the pulsed lasers and is arranged such that the gas which is the laser medium is excited by quickly discharging charge in a capacitor or the like to produce laser light. Generally, the duration of the laser light is several times 10 nanoseconds and the oscillation repeatedly occurs with a period of several milliseconds to several times of 10 milliseconds. For using such an excimer laser light source, which is a pulsed light source, in fine machining fields, an important problem is to control the emission energy per one pulse to an adequate value. In cases where the energy per one pulse is below a given threshold, a material to be machined is not changed at all irrespective of continuous illumination of the laser light so that the machining effect does not appear. On the other hand, when the pulse energy exceeds a predetermined value, portions other than the light-receiving portion are undesirably subjected to deformation, decomposition and deterioration. Particularly, in the case of machining, with the lithography technique, pattern lines of a super LSI device or the like whose width is below 0.5 micrometers which is substantially equal to the wavelength of the light from the light source, it is required to set the pulse energy of the laser light to an optimal value. In addition, the excimer laser has a characteristic that the output energy is monotonously lowered with long-time operation because of deterioration of the gas which is the laser medium, and the efficiency varies for a relatively short time due to generation of impurities and others. This requires a means to keep the pulse intensity to a set value by adjusting the output voltage of a high voltage power supply at all times.
Conventionally, such a pulsed light source is generally arranged such that the apparent pulse strength becomes constant by keeping the average output to a constant value, as disclosed in U.S. Pat. No. 4,611,270. A conventional pulsed light source will be described hereinbelow with reference to FIG. 1. In FIG. 1, a light-emitting section (pulsed laser) 1 intermittently emits light in response to supply of an electric energy from a power supply 2. A portion of light emitted from the light-emitting section 1 is derived through a half mirror or beam splitter 4 and then led to a light-receiving element (photodetector) 5. The light-receiving element 5 produces an electric signal corresponding to the intensity of the incident light thereon and supplies it through an averaging circuit 6 to a comparator 7. The comparator 7 compares a reference value Vr with the value of a signal corresponding to the laser output time-averaged in the averaging circuit 6 so as to control the power supply 2 in accordance with the difference therebetween. Secondly, operation of the conventional pulsed light source will be described hereinbelow with reference to FIG. 2. In FIG. 2, first, the light-emitting section 1 oscillates with a constant period as illustrated. When the efficiency of the light-emitting section 1 is lowered at time T1 so that the pulse energy P which is its output becomes lowered, the average output Pa also decreases, and hence the comparator 7 increases the power supply voltage V so that the difference .DELTA.V between the average output Pa and the reference value Vr becomes zero, thus thereby keeping the average value of the pulse energy P to a constant value.
However, such a conventional pulsed light source has a disadvantage in that difficulty is encountered in keeping the pulse energy at a constant value irrespective of variation of the light-emitting interval. In the actual machining, there is the possibility that a rest time in which the light-emission is stopped is made and the light-emitting period is changed in accordance with the nature of a material. In the case illustrated in FIG. 2, after time t2, the light-emission is intermittently effected with the rest time being made. When the rest time is made, the average output decreases, and therefore, the comparator operates to compensate therefor whereby the pulse strength immediately after the rest time becomes great. In the other case illustrated in FIG. 2 wherein the light-emitting period becomes longer after t3, since the average output is lowered the comparator operates so that the pulse energy becomes great. Particularly, in the case of using such a pulsed light source in a step-and-repeat type semiconductor circuit lithography apparatus, it is required to frequently change the light-emitting period. Thus, the conventional system causes generation of substandard products because of being difficult to keep constant the pulse energy. Moreover, the conventional control system is arranged to compensate for the output lowering due to relatively long time operation and generally arranged such that the time constant of the averaging circuit takes on the order of several seconds. When the time constant of the averaging circuit is set to be smaller, such problems do not occur. In such a case, however, a fluctuation of pulse energy is amplified, because the output energy of the excimer laser varies at every emission. Thus, in the case of the above-mentioned excimer laser with output variation on the order of several seconds, it is difficult with the conventional control method to maintain the output sufficiently constant.